The invention relates to the drying of moisture-containing materials, and although the invention is not restricted in its application to any particular type of material, it has been developed more especially for flexible sheet material such as fabric and paper or particulate material such as certain foodstuffs (e.g. grain, soya flour, coffee). Broadly, the invention can be used in any situation, where moisture can be removed from the material by the application of heat energy to the material. Consequently, the invention could be applied in some situations to the removal of moisture from a fluid.
The principal object of the invention is to improve the thermal efficiency of the drying process, thereby providing a means of reducing the energy consumption for a given reduction in moisture content of a given quantity of material.
When drying a moisture-containing material, it is usual to utilise one of two basic systems. The first system, which is very commonly used, can be termed the indirect drying method, wherein air, which is preferably heated, is passed over the material, and the moisture (or solvent) generally in the form of water vapour, is removed with the air stream.
The second drying system which is conventionally used, can be termed the direct contact system, and this method typically uses heated rollers, which contact a wet fabric or other material which is to be dried. The direct contact between the material and the heated rollers is arranged to produce an adequate increase in temperature of the material, sufficient to remove the moisture or solvent from the material which is to be dried.
In both the indirect and direct drying systems, external energy is generated to provide the air flow and/or the heat in the air or the rollers, and this energy is presented to the material which is to be dried. As a result, moisture leaves the material to be dried in the form of generated vapour, which contains the latent heat of evaporation, given up by the material to the moisture. Since the vapours given off by the material being dried contain the latent heat of evaporation which was required to convert them from a free liquid into a vapour or gas, then if those vapours are allowed to vent to atmosphere, this energy content is lost. A feature of the present invention is to provide an improved drying method and apparatus, by means of which at least part of the latent heat of evaporation of the vapour or gas produced in drying the material is re-utilised in the drying process.
In both the indirect and direct contact drying systems, the drying efficiency is greatly influenced by the amount of heat presented to the material which is to be dried, and it is necessary for this heat to be greater than the heat content of the moisture or solvent which is to be removed. Tests on conventional indirect drying systems usually show that the ratio of the heat energy which has to be supplied, to the energy of the moisture or solvents which are removed, is approximately 3:1. For the direct contact drying method, this ratio can be reduced to something in the order of 1.6:1. That is to say, in an indirect drying system, approximately 3000 B.T.U.s would be required to remove 1000 B.T.U.s in moisture or solvent, which is usually water, whereas in a direct drying system, 1600 B.T.U.s have to be supplied to remove solvent or moisture containing 1000 B.T.U.s heat energy. As will become more readily apparent below, it is a direct method that is desired in this invention, which means direct contact between the material and a solid heat transmitting medium or transmission of heat from a heat exchange located adjacent to the material.
It should be understood, that in any drying system, the basic requirements are (i) the creation of a temperature gradient so that the heat energy which is supplied flows to the moisture or solvent, and (ii) some method of effectively presenting the heat energy to the material which is to be dried. The present invention provides both a method and apparatus for fulfilling these basic requirements.